Photovoltaic (PV) power is a desirable complement to conventional electric power generation delivery systems. In assessing the value of photovoltaic power generation, electric power planners require solar energy resource data for specific areas where photovoltaic modules are to be installed. Solar energy reaches the earth surface along two paths-irradiance directly from the sun and diffuse irradiance from the sky. Both of those components are required in order to estimate the energy produced by PV systems in a given location. Direct normal and horizontal diffuse solar irradiance data and temperature data are used to estimate the energy produced by fixed and tracking PV systems. Those estimates support utility planning and PV system design studies by comparing the performances of various fixed and tracking array concepts. The data acquired can be used to compute the DC and AC power output for any fixed or tracking PV system at a given location.
Various forms of apparatus have been conceived for measuring solar radiation. Circa the year 1995 Ascension Technology Inc. (ATI), located in Waltham, Mass., introduced to the market a rotating shadowband pyranometer that was adapted to measure direct normal and horizontal diffuse irradiance. The ATI pyranometer is illustrated in FIGS. 1 and 2. It utilized a single light sensor 2 to determine total horizontal, direct normal and horizontal diffuse irradiance. The light sensor 2 was mounted on a bracket 4 attached to the end of a stationary tube 6 that was secured to a hollow stationary shaft 8 that in turn was affixed to one wall of a housing 10. The signal output of the light sensor was coupled to an electronic data logging system (not shown) by a cable (also not shown) that passed through tube 6 and shaft 8.
A shadowband in the form of a curved opaque strip 12 had one end attached to a member 14 that was secured to one face of a gear 16 that was mounted for rotation on shaft 8. Gear 16 meshed with a second gear 20 affixed to the output shaft 22 of an electric motor 24. The latter was mounted to a bracket 26 that had a pair of perforated ears 28 (only one of which is visible in FIG. 1) located along its opposite sides. Ears 28 that were used to pivotally mount the bracket to a yoke 30 mounted on a vertical post 32 that was fixed to the ground or to a building. Yoke 30 was rotatable on post 32 to permit it to be positioned to a selected azimuth position for optimum monitoring of sunlight.
Rotation of the shadowband was achieved by energizing motor 24, with operation of the motor being initiated and stopped by an electronic controller (not shown). The controller periodically caused the motor to rotate the shadowband unidirectionally through 360°, with rotation of the shadowband taking approximately one second. During that one-second interval the data logger sampled the pyranometer signal approximately 700 times. The sampling irradiance data was then analyzed to provide an estimate of the electrical power that could or should be produced by an existing or planned PV system. Typically the data logger and controller were mounted in a common box-like enclosure 36 that also contained a battery power supply (not shown) for the pyranometer, and a PV module 38 was attached to yoke 30 and connected to the battery so as to keep the latter charged by sunlight-derived electric power.
The ATI rotating shadow band pyranometer functioned well but suffered from the limitation that the mechanical structure for supporting and driving the shadow band was complicated and hence expensive. Friction in the gear system would cause it to wear out and bind over time. Also water penetration from rain affected the reliability and life of the drive system for the shadow band and/or associated electrical and electronic components, and repair or replacement of one or more components typically involved removal of substantially the entire drive system. Removal of the drive system included removal of the irradiance sensor which was assigned a unique calibration number. As a consequence of removing the drive system, the calibration number factor in the datalogger calculations needed to be adjusted.